Gate driver multi-chip module

ABSTRACT

A multi-chip module (MCM) provides power circuitry on a computer motherboard in a package of reduced size without sacrificing performance. The MCM co-packages essential power circuit components on a ball grid array (BGA) substrate. Two power MOSFETs disposed on the BGA substrate are connected in a half-bridge arrangement between an input voltage and ground. A MOSFET gate driver is electrically connected to respective gate inputs of the two power MOSFETs for alternately switching the power MOSFETs to generate an alternating output voltage at a common output node between the power MOSFETs. At least one Schottky diode is disposed on the BGA substrate and connected between the common output node and ground to minimize losses during deadtime conduction periods. The input capacitor of the circuit is contained within the MCM housing and is located close to the MOSFETs, reducing stray inductance in the circuit. The MCM package is thin and has dimensions of about 1 cm by 1 cm or less.

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/191,125 filed Mar. 22, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a multi-chip module (MCM). Morespecifically, the present invention relates to an MCM power circuit fora computer motherboard.

[0004] 2. Description of the Related Art

[0005] Power supply circuitry typically occupies a substantial area on acomputer motherboard. It would be desirable to reduce the size of thepower circuitry on a computer motherboard without sacrificingperformance.

SUMMARY OF THE INVENTION

[0006] The present invention provides an MCM which includes a MOSFETgate driver, two power MOSFETs, and associated passive elementsincluding an input capacitor all mounted on a ball grid array (BGA)substrate and packaged in a single chip.

[0007] The power MOSFETs of the MCM of the present invention areconnected in a half-bridge arrangement between an input voltage andground. The MOSFET gate driver is connected to respective gate inputs ofthe two power MOSFETs, and alternately switches the power MOSFETs togenerate an alternating output voltage at a common output node betweenthe power MOSFETs. At least one Schottky diode is disposed on the BGAsubstrate and connected between a common output node and ground tominimize losses during deadtime conduction periods.

[0008] The passive circuit components include an input capacitorconnected between the input voltage and ground which provides inputcapacitance for the converter. Advantageously, the input capacitor isphysically close to all other components. Additional components provideappropriate biasing for the gate driver. All components are encased in amolding compound to form the MCM package.

[0009] By mounting the input capacitor very close to other componentsand within the very small package, a number of advantages are realized,as follows:

[0010] First, there is a very low stray inductance between the inputcapacitor and the MOSFETs which reduces the “ring” that would be causedin the circuit including the MOSFET parasitic capacitance C_(OSS) andthe stray inductance L. Reducing the inductance reduces the circuitring.

[0011] Second, the location of the input capacitor within the MCMpackage provides layout independence for the mother board, which nolonger needs to contain that capacitor (at a distance from the MOSFETsin the MCM package).

[0012] Third, the capacitor acts as a bypass to conduction of unintendedcurrent (with a high di/dt) through the body diode of one of the MOSFETsin the package and acts to help clamp the Q_(RR) (reverse recoverycharge) of the MOSFET.

[0013] The module preferably is enclosed in a package that has sidedimensions of about 11 mm×11 mm (i.e., about 1 cm×1 cm) or less.Accordingly, the input capacitor is located less than 1 cm from theMOSFET.

[0014] The MCM of the present invention advantageously results in a 50%reduction in size with no performance trade off and is printed circuitboard (PCB) independent. The package advantageously provides aperformance increase over the discrete solution.

[0015] Other features and advantages of the present invention willbecome apparent from the following description of the invention whichrefers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a plan view drawing of the co-packaged active andpassive components in the MCM of the present invention.

[0017]FIG. 2 is an elevation view drawing of an MCM according to thepresent invention.

[0018]FIG. 3 is a circuit schematic of an MCM according to the presentinvention.

[0019]FIG. 3A is an equivalent circuit diagram of a portion of FIG. 3.

[0020]FIG. 4 is a timing diagram for an MCM according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Referring to FIG. 1, a diagram of a preferred layout for MCM 2 ofthe present invention is shown. MCM 2 includes six die mounted on a BGAsubstrate 4. A plurality of bonding pads 6 are disposed on the uppersurface of substrate 4.

[0022] Die 8 and 10 are power MOSFETs, preferably IRFC7811A andIRFC7809A power MOSFETs, respectively, mounted in a half-bridgeconfiguration. Die 12 is a MOSFET gate driver, preferably a Semtech SC1405 High Speed Synchronous Power MOSFET Smart Driver. Die 14, 16, and18 are Schottky diodes, preferably SKM863 diodes, connected as shown inthe circuit schematic of FIG. 3. The active components mounted on theupper surface of substrate 4 are connected electrically to correspondingbonding pads 6 using wire bonds 20.

[0023] The passive components shown in FIG. 1 include resistor R1, andcapacitors C1, C2, C3, and C4, also connected as shown in the circuitschematic of FIG. 3. The passive components are shown bonded directly tocorresponding pads 6. Significantly, capacitor C4 is mounted close toMOSFETs 8 and 10.

[0024] Referring to FIG. 2, MCM 2 of the present invention is shown inelevation. A plurality of solder balls 22 are arranged on the lowersurface of substrate 4. In the finished package, the components on theupper surface of substrate 4 are encapsulated in a mold compound 24 suchas Nitto HC 100. The dimension of housing 2 is about 1 cm×1 cm so itwill take very little space on a mother board.

[0025] Referring to FIG. 3, a circuit schematic of power supply MCM 2 isshown. Power MOSFETs 8 and 10 are mounted in a half-bridgeconfiguration, connected in series between an input voltage V_(IN) andground P_(GND). External circuit capacitance C_(EXT) is connected toV_(IN). A high-side output gate drive TG of MOSFET gate driver 12 isconnected to a gate input 20 of high-side power MOSFET 8. A low-sideoutput gate drive BG of MOSFET gate driver 12 is connected to a gateinput 22 of low-side power MOSFET 10. Gate driver 12 alternatelyswitches the power MOSFETs to generate an alternating output voltage ata common output node SW NODE between the power MOSFETs.

[0026] Schottky diodes 16 and 18 are connected between common outputnode SW NODE and ground to minimize losses during dead time conductionperiods. An input capacitor C4 is connected between the input voltageV_(IN) and ground P_(GND). The use of two parallel diodes 16 and 18helps in keeping a symmetrical layout of components. An output inductor30 generally will be connected to the SW NODE and to the output voltageterminal V_(OUT). An output capacitor C_(OUT) is also in the outputcircuit.

[0027] A supply voltage V_(DD) is provided to MOSFET gate driver 12 onpin V_(CC). A bootstrap circuit, consisting of Schottky diode 14, andresistor R1/capacitor C2 connected between the bootstrap pin BST and theDRN pin, is provided to develop a floating bootstrap voltage forhigh-side MOSFET 8.

[0028] A TTL-level input signal is provided on line DRV_IN to MOSFETdriver pin CO. Operation of the device is enabled by providing a minimumof 2.0 volts on enable pin EN of MOSFET driver 12. Status pin P_(RDY)indicates the status of the +5 V supply voltage. When the supply voltageis less than 4.4 V, this output is driven low. When the supply voltageis greater than 4.4 V, this output is driven high. This output has a 10mA source and 10 μA capability. When P_(RDY) is low, undervoltagecircuitry built into driver 12 guarantees that both driver outputs TGand BG are low.

[0029] Referring to FIG. 4, a timing diagram for MCM 2 is shown. A turnon delay t_(D(ON)) of typically 63 ns exists between the signal inputDRV_IN and output SW NODE of MCM 2. A turn off delay t_(D(OFF)) oftypically 26 ns exists between the signal input DRV_IN and output SWNODE of MCM 2. A portion of the delay is inherent in driver 12.

[0030] The supply voltage can range between 4.2 and 6.0 V. Inputvoltages of between 5 and 12 volts can be used, providing an outputvoltage range of 0.9-2.0 V. Output current is typically 15 A. The deviceoperates at frequencies from 300-1,000 kHz.

[0031] The operation of the circuit of FIG. 3 is considerably enhancedby the inherently close spacing between input capacitor C4 and MOSFET10.

[0032] First, the removal of capacitor C4 from the mother boardincreases layout flexibility for the mother board.

[0033] Second, since the capacitor C4 is very close to MOSFETs 8 and 10,the stray inductance in the circuit is reduced in comparison to thatwhich would be produced with C4 located outside the chip, on the motherboard. This close location (about one centimeter or less) substantiallyreduces the “ring” in the circuit. More specifically, as shown in FIG.3, MOSFET 10 has a parasitic capacitance C_(OSS). The circuit includingthe stray inductance L and C_(OSS) tends to ring at its resonantfrequency. By reducing L, the ring is also reduced.

[0034] A third benefit of capacitor C4 is that it clamps Q_(RR) (reverserecovery charge) of MOSFET 10 and keeps high di/dt from flowing out ofmodule 2 and into the mother board. More specifically, FIG. 3A is anequivalent circuit of portions of FIG. 3 showing in particular the bodydiode of MOSFET 10. During the dead time, during which both MOSFETs 8and 10 are off, conduction takes place through Schottky diodes 16 and 18of FIG. 3, but some “residual” current also is conducted through thebody diode of MOSFET 10. When MOSFET 8 turns on while the body diode ofMOSFET 10 is conducting, a reverse recovery current will be fed from theexternal capacitor C_(EXT) with very high di/dt. Capacitor C4, however,will act as a bypass to this high di/dt. The capacitor C4 of FIG. 3serves similar purposes.

[0035] Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.

What is claimed is:
 1. A multi-chip module (MCM) for providing powercircuitry on a computer motherboard, comprising: a ball grid array (BGA)substrate; two power MOSFETs disposed on the BGA substrate and connectedin a half-bridge arrangement between an input voltage and ground; aMOSFET gate driver disposed on the BGA substrate and electricallyconnected to respective gate inputs of the two power MOSFETs foralternatively switching the power MOSFETs to generate an alternatingoutput voltage at a common output node between the power MOSFETs; and atleast one diode disposed on the BGA substrate and connected between thecommon output node and ground to minimize losses during deadtimeconduction periods; and an input capacitor disposed on the substrate andconnected between the input voltage and ground.
 2. The module of claim1, further comprising another diode connected in parallel with the diodeconnected between the common output node and between the common outputnode and ground.
 3. The module of claim 1, wherein the substrate has anarea of about 1 cm by 1 cm or less.
 4. The module of claim 1, whereinthe input capacitor is spaced from the MOSFETs by less than 1 cm.
 5. Themodule of claim 1, wherein the input capacitor is located adjacent thefirst and second MOSFETs.
 6. The module of claim 1 which furtherincludes an insulation housing enclosing the substrate, the MOSFETs, thegate driver and the at least one diode, the ball grid array beingexposed through the bottom of the housing for mounting to a motherboard.
 7. The module of claim 6, wherein the housing has an area ofabout 1 cm by 1 cm or less.
 8. The module of claim 6, wherein the inputcapacitor is located adjacent the first and second MOSFETs.